The present invention relates to a solid state image sensor and, more particularly, to a solid state image sensor of a stacking type wherein a storage section and a transferring section are formed on a semiconductor substrate and a photo conductive film or layer is formed on the storage and transferring sections through an insulating layer.
A conventional solid state image sensor with a photo conductive film stacked thereon has good characteristics such as high sensitivity and low smear since light rays are photoelectrically converted by the photo conductive film. A solid image sensor of this type can be utilized in a variety of applications for TV cameras such as monitor TV cameras and has been developed as a solid state sensor of the next generation.
In a typical example of an image sensor, a photoelectric conversion section and a transparent electrode are sequentially formed on a substrate with a semiconductor device for transferring an electric charge. More specifically, the semiconductor device is formed on the substrate to transfer the charge, pixel electrodes connected to a diode region divided for every pixel or picture element are formed on the semiconductor substrate, a photoelectric conversion section is formed on the pixel electrodes, and a transparent electrode is formed on the photoelectric conversion section. The photoelectric conversion section blocks the injection of electrons therefrom, thereby constituting a multilayer structure with a barrier layer for providing a low dark current and small image lag. For example, when an i-type hydrogenated amorphous silicon is used to form a photoelectric film, p-type hydrogenated amorphous silicon carbide is often used as the barrier layer, thereby constituting an i-p structure. In order to smoothen the surface on which the photoelectric transducer section is formed, an insulating film can be formed on the surface of the semiconductor substrate.
In a conventional solid state image sensor of the type described above, blooming is a major problem. When a strong beam spot is formed on the image sensor, a reproduction region on the reproduction screen of a display apparatus vertically extends to greatly degrade image quality.
The primary cause for blooming is as follows. Incident light rays pass through a gap between pixel electrodes, become incident on the semiconductor substrate and are converted into carriers therein. The converted carriers are diffused into the transferring section and are mixed with the other carriers stored in the transferring section. In general, the total area of gaps between pixel electrodes is 30% of the total light reception area, so that blooming cannot be neglected. According to another cause for blooming, a charge storage capacity is several times the maximum vertical transfer charge. When strong light rays are incident on the image sensor, the generated signal charge is excessively large and overflows in the vertical transferring section.
A technique for preventing blooming described above is described in Japanese Patent Disclosure No. 55-104176. According to this prior art, an image sensor has a light-shielding section for shielding light at the gaps between pixel electrodes. However, it is apparent that blooming caused by transfer capacity cannot be prevented by only the provision of the light-shielding section. According to another conventional technique for preventing blooming, a voltage is applied to a transparent electrode to remove excessive charge to the transparent electrode. However, since there is a barrier layer under the transparent electrode for preventing injection of external charge, the amount of charge removed to the transparent electrode side is very small, and good blooming reduction effect cannot be expected. Furthermore, since the voltage is applied to the transparent electrode, a voltage actually applied to the photo conductive film is decreased. As a result, an image lag is formed and after image occurs.
According to still another technique for preventing blooming, an electrode connected to a photo conductive film is formed in a portion excluding the pixel area. An external voltage is applied to the electrode to remove the charge from the photo conductive film. Image sensors with a charge control electrode are described in Japanese Patent Disclosure Nos. 58-17784 and 58-80975. In the image sensor described in Japanese Patent Disclosure No. 58-17784, a charge control electrode is formed between the pixel electrodes to improve resolution. With this structure, blooming caused by light rays passing through a gap between the pixel electrode and the charge control electrode and incident on the semiconducor substrate cannot be sufficiently prevented. When packing density of pixels is increased, a pixel electrode width is decreased to about 1 .mu.m. It is thus difficult to form a charge control electrode in this electrode portion. In the image sensor described in Japanese Patent Disclosure No. 55-80975, a charge control electrode is arranged in a photoelectric conversion section. However, light shielding for the light rays incident on the semiconductor substrate is not sufficient as in the conventional image sensors described above, and blooming cannot be completely prevented. Furthermore, the process for forming an electrode in the photoelectric transducer section is complicated. As a result, the breakdown voltage at the photoelectric transducer section is decreased and may cause damage thereto.